Sweet Collage App
This is a great, basic collaging app. Lots of options, yet also lots of automation, so getting to your masterpiece is helped out. Some extras would be nice, like: ability to save file to come back and tweak later; greater control on where/how to split columns & rows... Sure, its not Photoshop, but for $4.99 its really great. (Best to try the free version first if in doubt.)
SlowX about
CollageIt Pro